The present invention concerns a ferritic stainless steel which can be used for ferromagnetic parts.
Ferritic stainless steels are characterised by a given composition, the ferritic structure being notably provided, after hot rolling and cooling of the composition, by a thermal annealing treatment conferring the said structure on them.
Amongst the major classes of ferritic stainless steels, defined notably according to their chromium and carbon content, there are:
the ferritic stainless steels which can contain up to 0.17% carbon. These steels, after the cooling which follows their production, have a two-phase austeno-ferritic structure. They may however be converted into ferritic stainless steels after annealing in spite of a relatively high carbon content;
the ferritic stainless steels whose chromium content is around 11 or 12%. They are fairly close to martensitic steels containing 12% chromium, but different through their carbon content, which is relatively low.
During the hot rolling of stainless steels, the structure of the steel can be two phase, ferritic and austenitic. If the cooling is, for example, energetic, the final structure is ferritic and martensitic. If it is slower, the austenite decomposes partially into ferrite and carbides, but with a higher carbide content than the surrounding matrix, the austenite having solubilised hot more carbon than ferrite. In both cases, a tempering or annealing must be performed on the hot-rolled and cooled steels in order to generate a completely ferritic structure. The tempering can be carried out at a temperature of approximately 820xc2x0 C. lower than the Ac1 alphaxe2x86x92 gamma transition temperature, which gives rise to a precipitation of carbides.
In the field of ferritic steels intended for an application using magnetic properties, the ferritic structure is obtained by limiting the quantity of carbides, and it is for this reason that the ferritic stainless steels developed in this field have a carbon content below 0.02%.
Steels are known which can be used for their magnetic properties, such as for example in the document U.S. Pat. No. 5, 769,974, which describes a method of manufacturing a corrosion-resistant ferritic steel able to reduce the value of the coercive field of the said steel. The steel used in the method is a steel of the resulfurated type. The sulfur reduces the cold deformation properties. The steel obtained by the method is therefore difficult to use for the production of cold-forged parts.
The patent U.S. Pat. No. 5,091,024 is also known, in which there are presented corrosion-resistant magnetic articles formed by an alloy consisting essentially of a composition with a low carbon content and a low silicon content, that is to say respectively below 0.03% and 0.5%. However, in the magnetic domain, it is important for the steel to contain a high silicon content in order to increase the resistivity of the material and to reduce eddy currents.
The purpose of the present invention is to present a stainless steel with a ferritic structure which can be used for magnetic parts with strong magnetic properties and presenting good properties of use in terms of cold forging and good machinability properties.
The object of the invention is a ferritic stainless steel which can be used for ferromagnetic parts which comprises, in its composition by weight:
0% less than Cxe2x89xa60.030%
1%xe2x89xa6Sixe2x89xa63%
0% less than Mnxe2x89xa60.5%
10%xe2x89xa6Crxe2x89xa613%
0% less than Nixe2x89xa60.5%
0% less than Moxe2x89xa63%
Nxe2x89xa60.030%
Cu xe2x89xa60.5%
Tixe2x89xa60.5%
Nbxe2x89xa61%
Caxe2x89xa71xc3x9710xe2x88x924%
0xe2x89xa610xc3x9710xe2x88x924%
Sxe2x89xa60.030%
Pxe2x89xa60.030%
the remainder being iron and the impurities inevitable from the production of the steel.
The other characteristics of the invention are:
the composition by weight also includes calcium and oxygen so that:
Ca greater than 30xc3x9710xe2x88x924%
O greater than 70xc3x9710xe2x88x924%
the ratio between the calcium and oxygen content Ca/O being
0.2xe2x89xa6Ca/Oxe2x89xa60.6
the steel contains inclusions of lime silico-aluminate of the anorthite and/or pseudo-wollastonite and/or gehienite type;
preferably the steel comprises, in its composition by weight:
0% less than Cxe2x89xa60.015%
1%xe2x89xa6Sixe2x89xa63%
0xe2x89xa6Mnxe2x89xa60.4%
10%Cr 13%
0% less than Nxe2x89xa60.2%
0.2%xe2x89xa6Moxe2x89xa62%
Nxe2x89xa60.015%
Cuxe2x89xa60.2%
Tixe2x89xa60.2%
Nbxe2x89xa61%
Caxe2x89xa630xc3x9710xe2x88x924%
Oxe2x89xa670xc3x9710xe2x88x924%
Sxe2x89xa60.003%
Pxe2x89xa60.030%
the remainder being iron and the impurities inevitable from the production of the steel;
preferably the steel comprises, in its composition by weight:
0% less than Cxe2x89xa60.015%
1%xe2x89xa6Sixe2x89xa63%
0xe2x89xa6Mnxe2x89xa60.4%
10%xe2x89xa6Crxe2x89xa613%
0% less than Nixe2x89xa60.2%
0.2%xe2x89xa6Moxe2x89xa62%
Nxe2x89xa60.015%
Cuxe2x89xa60.2%
Tixe2x89xa60.2%
Nbxe2x89xa61%
Caxe2x89xa730xc3x9710xe2x88x924%
Oxe2x89xa770xc3x9710xe2x88x924%
0.015xe2x89xa6Sxe2x89xa60.03%
Pxe2x89xa60.030%
the remainder being iron and the impurities inevitable from the production of the steel.
The invention also concerns a method of producing a ferritic steel wherein the composition by weight is subjected, after hot rolling and cooling, to a thermal annealing treatment and then a modification of cross-section of the drawing or stretch forming type.
The drawn or stretch-formed steel can subsequently be subjected to an additional recrystallisation annealing in order to perfect the magnetic properties of the part.